<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>add event listener</title>
  <style>
    .move_block {
      width: 200px;
      height: 200px;
      background: #ff3333 !important;
      position: absolute;
    }
  </style>
  <script>
    window.onload = function () {
      var obtn = document.querySelector('.move_block')

      obtn.addEventListener('mousedown', function (ev) {
        var events = ev || event
        console.log(ev)
      }, false)
    }
  </script>
</head>
<body>
  <div class="move_block"></div>
</body>
</html>